As illustrated schematically in FIGS. 1A and 1B, a common treatment for spinal stabilization is the fixation of two or more vertebrae 10, 12, performed by insertion of a pair of screws 14, 16, into each of the vertebrae to be fused and connecting the screw heads on either side of the spine by two rigid rods 17, 18. Cleaning the disc space 15 and inserting bone graft into the cleaned disc space causes bone to grow between the vertebrae until, until several months later, the fusion is completed. FIG. 1A is a cross sectional plan view of the superior vertebra 10, while FIG. 1B is a lateral view from the left of both fused vertebrae 10, 12.
The screws are usually inserted into the pedicles 19, two for each vertebra such that a minimum of four screws are required for each level of fusion. Spinal fusion by means of pedicle screw insertion is currently the most common procedure adopted for spinal stabilization, with hundreds of thousands of cases performed each year all over the world.
A different fixation technique, using only two obliquely inserted screws, one on either side of the spine, has also been described in the article entitled “Direct Pediculo-Body Fixation in Cases of Spondylolisthesis with Advanced Intervertebral Disc Degeneration”, by D. Grob et al published in European Spine Journal, Vol. 5, pp. 281-285; 1996. The surgical approach suggested in this article is for oblique transpedicular interbody fixation, and it was successfully performed at the L4-L5 and L5-S1 levels. In this technique, a pair of screws is inserted bilaterally through the pedicles of the inferior vertebra and passed diagonally across the disc space towards the anterior cortical rim of the superior vertebral body. FIG. 2A illustrates a lateral view of such a pair of vertebrae 20, 21, of a patient suffering from spondylolisthesis, showing the oblique entry of the screw 22, as described in Grob et al. Because of the anterior displacement of the upper slipped vertebra 20, the entry angle of the screw is closer to the lateral plane than 45°, the significance of which will be described hereinbelow. Grob et al also describes the use of an inward angle of 5° to 10° in the saggital direction, as will be shown in FIGS. 2B and 2C below, to ensure that the screws remain within the body of the superior vertebra 20 and do not penetrate the cortical bone thereof.
Grob et al describes the cases of 16 patients with average follow-up of 31 months (24-77 months) treated with this direct pediculo-body fixation technique. Clinical evaluation showed significant reduction in pain and increase in functionality. Radiologic evaluations indicate solid bony fusion in all cases, and no neurological or other complications were observed. The stand-alone two-screw construction was concluded to be simple to implement and clinically successful. The screws provided three-dimensional stability, which led to bony unions and favorable clinical outcomes in all patients. This procedure thus uses only two screws, rather than four screws and two rods.
Even though this procedure was performed with good success on a significant number of patients (16), the technique has not gained much acceptance in the operating room. One reason for its low acceptance may be that the required screw trajectories pass close to nerve roots, and hence a clear view of the operation site is needed to minimize the risk of damage to a nerve, whether at the spinal canal or at the foramen. This required, as described in Grob, an open surgical procedure with a large incision to expose the entire region of the oblique trajectory from the skin to the entry point at the vertebra, and towards the second vertebrae into which the screw is inserted, so that the surgeon is able to estimate accurately the correct entry position and angle. This technique was therefore highly traumatic to the tissues and muscles of the back, and this may have contributed to the lack of acceptance of the technique, despite its structural simplicity.
In this respect the procedure is different from the common spinal fusion methods using four screws per level. As illustrated in FIGS. 1A and 1B, such prior art spinal fusion methods involve a screw trajectory which remains exclusively within the vertebral bone, from the entry point at the pedicle through to the vertebral body. Unless a gross error has been made in the insertion trajectory, there is little danger of nerve damage. It is reported that only about 3% of such operations result in permanent neurological deficits with this technique. The insertion trajectory can either be determined visually by the surgeon, or can be performed robotically, based on an operative plan using pre-surgical CT images, or by use of a navigation system to define an accurate path.
In addition, because of the difficulty of safe insertion of the screws, as described in Grob, it is necessary to perform the oblique drilling under fluoroscopic control, which may involve both the patient and the operating room staff with unnecessarily significant levels of X-ray exposure.
The procedure described in Grob was performed on patients suffering from spondylolisthesis, involving significant anterior slippage of the superior vertebra and an advanced stage of disc resorption with a reduction of disc height by at least 75% of the original height. Under these conditions, and as shown in FIG. 2A, the drill enters the superior vertebra through the posterior end plate, and at an angle of less than 45° to the lateral plane, thus clearly avoiding the foramen 27. However, if the procedure were to be performed on a patient having normal vertebral alignment and a normal disc height, the entry angle would need to be tilted closer to the axial direction, thereby involving a closer encroachment to the nerve roots at the foramen. This would increase the risk of nerve damage in performing this oblique entry procedure. Furthermore, the size of the incision that has to be made in the subject's back is considerably longer for a normally aligned vertebral spine, than for a patient with spondylolisthesis, since the angle of entry of the drilling axis is closer in the case of the normally aligned spine to the axial direction of the spine. This makes the open surgery approach even less inviting as a technique for treating aligned vertebrae. Finally, it should be noted that in a significant number of patients, the vertebrae may lie several centimeters below the surface of the patient's skin, beneath layers of fat and muscle tissue, such that the additional depth from the skin to the vertebra, in combination with the angle of the trajectory to the normal, would increase the length of the incision needed even more than indicated above.
This oblique entry procedure has been described again recently, in US patent publication number US 2009/0163957 to S. St. Clair et al, for use in fusion procedures in subjects having normal vertebral separation. FIGS. 2B and 2C illustrate the position and path of entry of such a pair of obliquely inserted screws. Though the vertebral alignment in FIGS. 2B and 2C is different from that in FIG. 2A, similar items are similarly numbered to those of FIG. 2A. FIG. 2B shows schematically a posterior view of the adjacent vertebrae 20, 21, with interbody oblique fixation screws 22, showing the inward tilt of the screws as described in Grob et al., and FIG. 2C shows a lateral view of the same vertebrae. The drawings, and FIG. 2C in particular, show the path of the screws from the inferior articular process 23 of the facet joint of the inferior vertebra 21, traversing the pedicle and through the endplate 24 of the inferior vertebra, across the interbody space 28 between the vertebrae, through the inferior endplate 25 of the superior vertebra body 20, through the centrum of the superior vertebra and towards the junction 26 of the superior endplate and the anterior vertebral surface of the superior vertebra. It is observed in FIG. 2C that the entry angle in the posterior-anterior plane is at an angle of 45° or less to the longitudinal axis of the spine defined by the superior and inferior vertebrae, such that the drill trajectory passes significantly closer to the position of nerve roots at the foramen 27 than was the case with the procedures described by Grob, performed on spondylolisthesic patients. The procedures described in the US 2009/0163957 publication therefore further emphasizes the need for an apparatus and method for performing oblique stabilization or fusion more safely than the Grob prior art procedures, where only spondylolisthesic patients were treated.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.